A high speed turbo machine, such as, for example, a steam or gas turbine, generally comprises a plurality of blades arranged in axially oriented rows, the rows of blades being rotated in response to the force of a high pressure fluid flowing axially through the machine. Due to their complex design, natural resonant mechanical frequencies of the blades may coincide with or be excited by certain blade rotational speeds and rotational harmonics thereof. To prevent excessive vibration of the blade about its normal position, prudent design practice dictates that the blades be constructed such that the frequencies of the lowest modes fall between harmonics of the operating frequency of the turbine. In addition, the blades may be excited by non-synchronous forces such as aerodynamic buffeting or flutter. In order to avoid the vibration exceeding certain levels and setting up objectionable stresses in the blades, it is common to monitor the vibrations of the blades, both during the design and testing of the turbine and during normal operation of the turbine. For example, it is known to use non-contacting proximity sensors or probes to detect blade vibrations. The probes detect the actual time-of-arrival of each blade as it passes each probe and provide corresponding signals to a blade vibration monitor system (BVM). Small deviations due to vibration are extracted, from which the BVM may determine the amplitude, frequency, and phase of the vibration of each blade.
Many blade tip vibration monitors employ multiple sensors in order to provide multiple blade pass signals each revolution of the blades to remove frequency foldover that is inherent in single sensor blade tip instruments, and to measure synchronous vibration. However, when multiple sensors are used, there is typically no way to ensure that all of the sensors are exactly coplanar with the blade row. That is, the sensors may be misaligned relative to each other axially in a direction parallel to the axis of rotation of the blades, such that the sensors do not all sense the same location on the blade tip. Thus, while the blade pass signal is sampled multiple times each revolution, there is an error that is introduced in this sampling due to misalignment of the sensors and slight variances in the electrical performance of each sensor. This results in an error and spectral noise in the measurement.